Selective hydrogenations of unsaturated hydrocarbon compounds are of high industrial significance. The pyrolysis of naphtha for the production of ethene, propene, butanes, 1,3-butadiene and aromatics is a key process in the modern petrochemical industry. For the nearly complete removal of alkynic compounds from the C2, C3 and C4 cuts, selective hydrogenations are generally used.
For instance, the hydrogenation of acetylene is an important industrial process to remove traces of acetylene in the ethylene feed for the production of polyethylene. Because acetylene poisons the catalyst for the polymerisation of ethylene to polyethylene, the acetylene content in the ethylene feed has to be reduced to the low ppm range. Moreover, economic efficiency requires high selectivity of acetylene hydrogenation in the presence of an excess of ethylene to prevent the hydrogenation of ethylene to ethane.
Typical hydrogenation catalysts contain palladium dispersed on metal oxides. While palladium metal exhibits high activity, e.g. in the hydrogenation of acetylene, it possesses only limited selectivity because of the formation of ethane by total hydrogenation and C4 and higher hydrocarbons by oligomerisation reactions.
The C3 cut (propylene) is generally purified by selective hydrogenation of propyne (methylacetylene) and propadiene (allene), and the obtained propylene may be further processed to polypropylene.
Another important selective hydrogenation in industry is the removal of traces of 1,3-butadiene from the C4 fraction after the extractive separation thereof. Pd/Al2O3 catalysts are commonly used in this reaction. Furthermore, the selective hydrogenation of 1,5-cyclooctadiene, obtained by cyclic dimerization of 1,3-butadiene, to cyclooctene on Pd/Al2O3 and of benzene to cyclohexene on ruthenium catalysts are of importance.
In all of these selective hydrogenations, further improvements of the selectivity to the desired product and an increased long term stability of the used catalyst have been strongly desired.
The use of ordered intermetallic compounds as catalysts in a variety of different reactions is generally described in US 2004/0126267 A1 and WO 2004/012290 A2. However, these documents fail to disclose the application of this type of compounds to hydrogenations, let alone selective hydrogenations. In fact, the focus of these references is on their use in fuel cells. As regards the form of the catalyst, US 2004/0126267 A1 also provides for the presence of the catalyst bed in the form of a granular powder, coated beads or a coated ceramic monolith.
The intermetallic compounds PdGa or Pd3Ga7 are described by E. Hellner et al. in Z. Naturforsch. 2a, 177-183 (1947) and by K. Khalaff et al. in J. Less-Common Met. 37, 129-140 (1974). Recently, K. Kovnir et al. in Stud. Surf. Sci. Catal., 162, 481-488 (2006) uncovered the potential of these materials as highly-selective catalysts for the acetylene partial hydrogenation. In the catalytic tests, the authors used unsupported intermetallic compounds obtained by melting the necessary amounts of palladium and gallium. Furthermore, samples obtained by milling the as-made compounds in a swing mill, and samples obtained by subjecting the as-made materials to chemical etching using aqueous ammonia solution were tested. While the activity of the as-made samples was not satisfactory, it could be enhanced by the milling and etching treatment. M. Armbrüster et al., Z. Anorg. Allg. Chem. 632, 2083 (2006) provides for a similar disclosure. However, in the case of milling, the achieved increase in activity still left room for improvements. On the other hand, the etching treatment involves a time consuming after-treatment of the as-made samples by stirring in diluted ammonia solution.
In the scientific literature, there are only few articles dealing with the mixing of catalysts with silica. For instance, A. M. Youssef et al. in Appl. Catal. A, 81, 1-13 (1992) describe the mechanical mixing of precipitated, non-washed silica hydrogel and hydrous magnesium hydroxide. The respective slurries were thoroughly mixed by mechanical stirring, filtered, washed and finally dried at 100° C., prior to thermal treatment at 500, 600 and 800° C. The obtained preparations were tested in the heterogeneous catalytic decomposition of isopropanol.
C. Wögerbauer et al., in J. Catal., 201, 113-127 (2001) describe mechanical mixtures of Ir black or IrO2 with various mixing materials like silica xerogel, alumina and H-ZSM-5. The mixtures are claimed to show outstanding DeNOx activity compared to their supported counterparts. Moreover, Pt black, Pd black and Rh black, each mixed with silica xerogel were used in the reduction of NO with propene.
The mechanical mixing and grinding of Ag nanoparticles and SiO2 powder to form a uniform mixture is described in Z. Qu et al. in Catal. Today, 93-95, 247-255 (2004). Following calcination pretreatment with oxygen at 500° C. or higher, the materials were used in CO selective oxidation. Mechanical Ag/SiO2 mixtures without calcination showed practically no activity.
In view of the above prior art, it is an object of the present invention to provide a process for the selective hydrogenation of unsaturated hydrocarbon compounds, in particular of ethyne (acetylene) in admixture with a large excess of ethene (ethylene) to afford ethene, which is further improved in terms of activity while maintaining a high selectivity, and moreover uses catalysts that can be prepared easily. It is another object to provide alternative catalysts comprising ordered intermetallic compounds having enhanced activity.